212 MICROSCOPICAL TECHNIQUE. 



lo per cent. : — Nitrate of urea, 4 to 5 grm. per litre, Thus pre- 

 pared, the liquids may be sterilised at 100 deg. in cultivation flasks. 

 It is hardly necessary to point out that nitrate of urea should 

 not be used to prevent the coagulation of albumen if the experi- 

 ments relate to nutrition or fermentation of matter containing 

 albumen. 



Preparing and Staining Yeast."*^— For fixing preparations of 

 yeast, Dr. H. Moeller uses a i per cent, solution of iodide of 

 potassium saturated with iodine, this fluid ten times diluted, and 

 also iodine-water. The material and the fixative may be mixed 

 together at once or upon the cover-glass, which merely requires a 

 smear. When fixed and dried the preparation must be thoroughly 

 hardened. This may be done by leaving the preparations in the 

 iodine solution for a day, and then, after washing in water and 

 weak spirit, keeping them in absolute alcohol for one or two days. 

 The time required for hardening may be diminished by repeatedly 

 boiling the alcohol, and the preparations are more clearly stained 

 if they are then immersed in chloroform for a day. It is always 

 useful to pass the cover-glasses once or twice through the flame. 



The preparations are best stained by means of hsem.atein and 

 picric acid, the latter acting as a mordant. But it is essential that 

 the preparations should be thoroughly fixed and hardened ; they 

 may then be treated with a saturated aqueous solution of picric 

 acid for from half-an-hour to three hours. The preparation is then 

 passed through water so as to wash off some, but not all, of the 

 picric acid. For staining, an alkaline solution of haematoxylin is 

 used. It would not appear, however, that the foregoing staining 

 was more advantageous than that with aniline, of which the fol- 

 lowing were successfully employed : — Phenol-fuchsin, alkaline, 

 methylen-blue. Gram's method, and also gentian-violet in carbolic 

 acid, water, glycerin, i per cent, acetic acid, and i per cent, iodide 

 of potash. 



If the anilin dyes are used, the preparation should be over- 

 stained and then differentiated by some decolorant ; if Gram's 

 method be adopted, alcohol must be used ; but for other stains a 



*Journ. R. Micr. Soc, 1893, pp. 1 18 — 1 19 ; from the Centralb.f. Bakteriol. 

 u. Parasitenk., xii. (1892), pp. 537 — 50. 



